Mucus secretions from the female reproductive tract are extremely difficult to penetrate even by virus-sized objects. Why then do these barriers sometimes fail to protect against sexually transmitted pathogens? Our pilot data suggests some pathogens are too large to penetrate mucus mesh-spacing (pore size), and mucus also traps some pathogens like fly-paper before they can reach target cells. In contrast, abnormal mucus may not be so protective. About 1 in 3 women have bacterial vaginosis (BV), and they are at >2-6-fold increased risk of infection by HIV and other viral, bacterial, and protozoan pathogens. BV is a polymicrobial overgrowth of vaginal flora that decimates healthy lactobacilli, transforms vaginal mucus to a watery secretion, and partially eliminates the acidity of the vagina. We have developed mucus penetrating particles (MPP) that reveal the mucus mesh-spacing in fresh human samples. We can thus identify pathogens that are slowed in ex vivo genital mucus fluids by steric occlusion (pathogen size > mesh-spacing) and/or trapped by adhesion. Our pilot observations indicate native (acidic) cervicovaginal mucus from women with healthy vaginal flora adhesively traps HPV, HIV, and HSV. But, if partially neutralized with NaOH to mimic neutralizing actions of BV, HIV and HSV can penetrate this mucus barrier. Our overall hypothesis is that mucus secretions from women with healthy vaginal flora are highly protective against pathogen penetration, but that this protection is greatly diminished in women with BV. To test this, Aim 1 will extend our pilot observations and include all mucus secretions through which STD pathogens are usually transmitted: endocervical mucus, healthy cervicovaginal mucus, BV secretions, and semen. We will use multiple particle tracking and MPP to characterize the meshspacing and protective viscoelastic properties of these genital secretions at the nano- to micro- length scales experienced by pathogens. Aim 2 will identify the secretions that can block or retard penetration by major STD pathogens (HIV, HSV, HPV, N. gonorrhoeae and C. trachomatis), and secretions that are rapidly penetrated. We will also determine the exact viscosity and elasticity experienced by each pathogen in the various human secretions. This will provide important quantitation of the extent of impediment each pathogen experiences. Taken together, the results from Aims 1 & 2 will also distinguish pathogens blocked sterically, and those trapped adhesively, in each secretion. Aim 3 will investigate the roles of lactic acid, and lactate anions, in causing STD pathogens to be trapped by mucus. Our pilot results show lactic acid specifically alters the surfaces of HIV and HSV, and this may help explain how they are trapped by healthy mucus. This project will likely clarify how abnormal mucus secretions caused by BV increase susceptibility to penetration of a broad range of viral, bacterial, and protozoal pathogens. The results may also support the creation of new methods that use lactic acid to help prevent BV and enhance the protective effects of mucus secretions